Sheet Feeding Device and Image Forming Device

ABSTRACT

A sheet feeding device includes: a sheet stacking unit that stacks a bundle of sheets; a pick-up roller that is enabled to move up and down and sends a sheet of the bundle of sheets toward downstream side in a carrying direction; a pair of rollers that is arranged at downstream side in the carrying direction, and when multiple sheets are sent out by the pick-up roller, that separates the first sheet from the second sheet or later; a separation member that is arranged between the pick-up roller and the pair of rollers and that is in contact with a leading end of the sheet and limits the number of the sheet carried to the pair of rollers; and a hardware processor that determines a height position of the pick-up roller corresponding to a type of the sheet and sets the pick-up roller to be at the determined height position.

Japanese patent application No. 2020-130119 filed on Jul. 31, 2020 including description, claims, drawings, and abstract the entire disclosure is incorporated herein by reference in its entirety.

BACKGROUND Technological Field

The present invention relates to a sheet feeding device and an image forming device.

Description of the Related Art

Image forming devices such as printers or MFPs (Multifunction Peripherals) include sheet feeding devices that store a bundle of sheets such as print papers, and feed and carry each sheet of the bundle of sheets. This known technique is introduced for example in Japanese Patent Application Laid-Open No. JP 2005-187145 A. According to the known technique, the sheet feeding device includes a sheet storage in which the bundle of sheets is stored and a pick-up roller that sends the sheet toward downstream in a carrying direction. The pick-up roller is arranged at an upper side of the sheet storage. The pick-up roller is in contact with the sheet that is placed at an uppermost of the bundle of sheets and rotates so that the sheet is sent out toward downstream in the carrying direction.

It is possible that the pick-up roller sends multiple sheets which are placed at an upper part of the bundle of sheets at the same time. In order to solve this problem that the multiple sheets are sent at the same time, a pair of rollers that is formed from a sheet feeding roller and a separation roller is placed at downstream side of the pick-up roller. After the multiple sheets are sent by the pick-up roller, the pair of rollers feeds the first sheet at the uppermost toward downstream in the carrying direction and restricts the sheets that are the second one and the followings to be fed toward downstream in the carrying direction.

When the multiple sheets are sent at the same time by the pick-up roller, it may not be possible to stop feeding of all the sheets that are the second one and the followings. In this case, more than one sheet that is placed in the upper part is fed toward downstream of the pair of rollers among the multiple sheets sent by the pick-up roller. This causes an occurrence of a jam. In order to prevent the occurrence of this type of jam, the conventional sheet feeding device is provided with a separation member that has an inclined surface in the carrying direction between the pick-up roller and the pair of rollers. A leading end of the sheets multifed by the pick-up roller is pressed to the inclined surface of the separation member so that multifeeding of the multiple sheets to the pair of rollers is restricted.

The separation member effectively prevents the multifeeding of the multiple sheets to the pair of rollers if the sheet is a thin paper. To be more specific, when the sheet is the thin paper, the separation member only feeds two to three sheets in the upper layer of the multiple sheets sent out by the pick-up roller along the inclined surface and successfully feeds the sheets to downstream of the pair of rollers.

On the other hand, the sheet might be a thick paper that has a rigidity equal to or higher than a predetermined level. In this case, the separation member does not function normally. To be more specific, when the sheet is the thick paper, a leading end of the sheet that is sent by the pick-up roller gets in contact with the separation member. When the leading end of the sheet gets in contact with the separation member, large carrying resistance is applied to the sheet by the separation member. The pick-up roller, therefore, cannot carry the sheet toward downstream along the inclined surface, and a carrying failure can be easily occurred. The angle of the inclined surface of the separation member is put close to parallel to the carrying direction so that the carrying resistance of the sheet such as the thick paper can be less. Although this may prevent the occurrence of the carrying failure, a multifeeding prevention function that is effective for the thin paper is lowered.

In general, the thin paper or a plain paper is more frequently used compared to the thick paper with the image forming device. It is necessary to set the angle of the inclined surface of the separation member in order to prevent the multifeeding of the multiple sheets when the thin paper or the plain paper is to be used. As a result, when the sheet such as the thick paper that has a rigidity higher than a predetermined level, the carrying failure can be easily occurred.

Also, when a regulating member such as a manual bypass tray cannot be provided on a rear end side of the sheet, a structure that stores while the rear end side of the sheet is pulled up by the sheet stacking part with an angle in order to prevent the sheet moving backward in response to the sheet feeding operation is commonly used. It is easier to move the sheet toward the pair of rollers at downstream side if the sheet has an angle close to horizontal in the lower position of the pick-up roller. The sheet stacking part supports the leading end side and the rear end side of the sheet in the different angle. The thin paper or the plain paper has the low rigidity, so the bundle of sheets is stored in a shape along with the angle of the sheet stacking part. On the other hand, if the sheet is the thick paper, only the leading end and the rear end of the sheet are in contact with the bottom surface of the sheet stacking part. The center part of the sheet is floated and stacked. When the sheet is sent out by the pick-up roller, the sheet is no longer parallel to the carrying direction, and is in contact with the inclined surface of the separation member in an angle close to right angle. In this case, much larger carrying resistance is applied.

Furthermore, the pick-up roller applies a certain pressing pressure while being in contact with the top surface of the bundle of sheets so that the sheet is sent to downstream side in the carrying direction. If the sheet is the thick paper and the center part of the sheet is floated from the bottom surface, the sheet is bent so that the pressing pressure by the pick-up roller is absorbed. The friction force necessary for carrying the sheet cannot be obtained and the carrying failure is occurred.

SUMMARY

The present invention is intended to solve the above problems. Thus, the present invention is intended to provide a sheet feeding device and an image forming device that correspond to multiple kinds of sheets varying from thin papers to thick papers and ensure a feeding function even when any kind of sheet is used.

First, the present invention is directed to a sheet feeding device.

To achieve at least one of the abovementioned objects, according to an aspect of the present invention, the sheet feeding device reflecting one aspect of the present invention comprises: a sheet stacking unit that is enabled to be lifted and lowered while a bundle of sheets is stacked; a pick-up roller that is in contact with a top surface of the bundle of sheets and is enabled to move up and down in accordance with the lifting and lowering movement of the sheet stacking unit, and that sends a sheet of the bundle of sheets toward downstream side in a carrying direction; a pair of rollers that is arranged at downstream side in the carrying direction of the pick-up roller, and when multiple sheets are sent out by the pick-up roller, that separates the first sheet from the second sheet or later and feeds the first sheet toward downstream side in the carrying direction, and restricts a movement of the second sheet or later; a separation member that is arranged between the pick-up roller and the pair of rollers and that is in contact with a leading end of the sheet that is sent out by the pick-up roller and limits the number of the sheet carried to the pair of rollers; and a hardware processor that determines a height position of the pick-up roller corresponding to a type of the sheet stacked on the sheet stacking unit and lifts and drives the sheet stacking unit to set the pick-up roller to be at the determined height position.

Second, the present invention is directed to an image forming device including the sheet feeding device.

BRIEF DESCRIPTION OF THE DRAWING

The advantages and features provided by one or more embodiments of the invention will become more fully understood from the detailed description given herein below and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention.

FIG. 1 illustrates an exemplary structure of an image forming device which includes a sheet feeding device;

FIG. 2 illustrates a side-view of an exemplary structure of the sheet feeding device that is arranged as a manual bypass tray;

FIG. 3 illustrates a side-view of an exemplary structure of the sheet feeding device when a sheet such as a thin paper is fed by a pick-up roller;

FIG. 4 illustrates an enlarged view of the pick-up roller, a pair of rollers and a separation member;

FIG. 5 illustrates a side view of the sheet feeding device with a sheet stacking unit on which the sheet that has high rigidity such as a thick paper is stacked;

FIG. 6 illustrates an example when the pick-up roller is in contact with the sheet such as the thick paper;

FIG. 7 illustrates an enlarged view when a height position of the pick-up roller reaches the first position while the sheet such as the thick paper is stacked;

FIG. 8 illustrates an enlarged view of the pick-up roller, the pair of rollers and the separation member when the sheet that has height rigidity such as the thick paper is fed;

FIGS. 9A, 9B and 9C illustrate an example of a change in the pressing force of a pressing member;

FIG. 10 illustrates a block diagram showing an example of a control structure of the image forming device;

FIG. 11 illustrates a flow diagram explaining an exemplary procedure of the process that controls operations of the sheet feeding device; and

FIGS. 12A, 12B and 12C illustrate examples of the screens displayed on a display unit of an operational panel.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, one or more embodiments of the present invention will be described with reference to the drawings. However, the scope of the invention is not limited to the disclosed embodiment.

FIG. 1 illustrates an exemplary structure of an image forming device 1 in which the present invention may be practiced. The image forming device 1 is constructed as one of MFPs that include multiple functions including, for example, copy function, scan function and print function. The image forming device includes an image forming unit in the center part of the device body and a feeder 3 with multiple paper feeding trays 3 a, 3 b, 3 c and 3 d in a lower part of the image forming unit 2. A bundle of sheets formed from stacked sheets such as printing papers is stored in each of the multiple paper feeding trays 3 a, 3 b, 3 c and 3 d. The feeder 3 feeds the sheet from one of the multiple paper feeding trays 3 a, 3 b, 3 c and 3 d and supplies to the image forming unit 2. The image forming unit 2 forms an image on the sheet fed by the feeder 3 in electrophotography, for instance, and ejects the sheet on which the image is printed on a paper discharging tray 2 a in an upper part.

The image forming device 1 is provided with a manual bypass tray 4 at a side of the device body. The manual bypass tray 4 is openable and closable to the side of the device body. FIG. 1 illustrates the manual bypass tray 4 when it is opened and used in the tilting posture. The manual bypass tray 4 is rotated and is in a standing posture when it is not being used so that the manual bypass tray 4 is accommodated in a recessed part 7 formed at the side of the device body.

The manual bypass tray 4 is one of sheet feeding devices 5 of the present invention. The manual bypass tray 4 is provided with a sheet stacking unit 6 on which a bundle of sheets that includes at least one sheet such as the printing paper can be stacked when the manual bypass tray 4 is in use as illustrated in FIG. 1. A space 8 is formed at the lower side of the recessed part 7 formed at the side surface of device body. A leading end of the bundle of sheets that is stacked on the sheet stacking unit 6 can be inserted in the space 8. The leading end of the bundle of sheets is inserted in the space 8 and is stacked on the sheet stacking unit 6. The manual bypass tray 4 feeds each sheet of the bundle of sheets stacked on the sheet stacking unit 6 one by one and feeds to the image forming unit 2 inside the device body. The image forming unit 2 is capable of forming an image not only on a sheet fed from the feeder 3 but also on a sheet fed from the manual bypass tray 4.

The image forming device 1 includes an image reader 10 and an automatic document feeder (ADF) 11 in an upper part of the device body. The image reader 10 optically reads an image of a document placed by a user and generates image data. The automatic document feeder 11 includes a document tray 12. The automatic document feeder 11 automatically carries each sheet of the document placed on the document tray 12 to a reading position of the image reader 10. The image reader 10 is constructed to carry out a reading operation synchronous with a document carrying operation carried out by the automatic document feeder 11. The image reader 10 reads an image of the document when the sheet of the document carried by the automatic document feeder 11 passes through the predetermined reading position.

Furthermore, the image forming device 1 is provided with an operational panel 15 at a front side of the device body. The operational panel 15 is a user interface for the user. The operational panel 15 displays various types of screens operable for the user and accepts operations from the user such as a job setting operation or a job processing instruction. For a copy job, for instance, the image forming 1 receives a copy job setting operation by the user. The image forming device 1 determines a tray from which a sheet is fed from among the multiple paper feeding trays 3 a, 3 b, 3 c and 3 d and the manual bypass tray 4 based on the user's setting operation. Once the user instructs to start processing of the copy job, the image forming device 1 drives the image reader 10 and the automatic document feeder 11 to start the document reading operation, and carries out an operation to feed the sheet from the bundle of sheets stacked on the tray specified by the user one by one. The image forming device 1 then processes the copy job.

FIG. 2 illustrates a side-view of an exemplary structure of the sheet feeding device 5 that is arranged as the manual bypass tray 4. FIG. 2 illustrates an example when a sheet that has low rigidity such as a thin paper or a plain paper is stacked on the sheet stacking unit 6. The sheet feeding device 5 includes a pick-up roller 30, a position detecting sensor 38, a pair of rollers 40 and a separation member 50. The sheet feeding device 5 picks up a sheet which is placed at the top of a bundle of at least one sheet 9 stacked on a top surface of the sheet stacking unit 6 and feeds toward a carrying path 45 which is downstream side in the carrying direction. The carrying direction of the sheet in FIG. 2 is left direction.

The sheet stacking unit 6 includes a sheet rear end supporting unit 21, a sheet leading end supporting unit 22 and a wall 23. The sheet rear end supporting unit 21 is tilted from the side surface of the image forming device 1. The sheet leading end supporting unit 22 is arranged inner side of the image forming device 1. The wall 23 aligns the leading end of the bundle of sheets 9 with an end of the sheet leading end supporting unit 22. The sheet leading end supporting unit 22 is, for instance, arranged in a bottom of the space 8 to which the end (the leading end) of the sheet leading end supporting unit 22 is inserted. The wall 23 is erected at the end (the leading end) of the sheet leading end supporting unit 22. When the leading end of the bundle of sheets 9 is butted against the wall 23, the leading end position of the bundle of sheets 9 is aligned.

The sheet leading end supporting unit 22 is formed from a plate formed member arranged in a nearly horizontal direction, for example. The sheet leading end supporting unit 22 supports the leading end of the bundle of sheets 9 nearly horizontally. The sheet rear end supporting unit 21 formed as an inclination part that has a certain angle against the sheet leading end supporting unit 22 supports the rear end of the bundle of sheets 9. To be more specific, in order to avoid retracting of the sheet in response to the feeding operation when the sheet is continuously fed, the sheet rear end supporting unit 21 is in a tilted posture that has the certain angle against the nearly horizontal sheet leading end supporting unit 22. The sheet rear end supporting unit 21 puts the rear end side of the sheet up and supports. The sheet rear end supporting unit 21 includes a supplemental supporting unit 21 a. The supplemental supporting unit 21 a can extend a part to support the rear end of the bundle of sheets 9 when the sheet size is big. The supplemental supporting unit 21 a can be stored in the sheet rear end supporting unit 21.

When the sheet that has low rigidity such as the thin paper is placed on the sheet stacking unit 6, the lower surface of the leading end side of the bundle of sheets 9 is supported by the sheet leading end supporting unit 22, and the lower surface of the rear end of the bundle of sheets 9 is supported by the sheet rear end supporting unit 21. More specifically, the bundle of sheets 9 that has low rigidity is bended in a part that joints the sheet leading end supporting unit 22 and the sheet rear end supporting unit 21 and is stacked, thus the leading end part of the bundle of sheets 9 is supported nearly horizontally by the sheet leading end supporting unit 22.

The sheet stacking unit 6 on which the bundle of sheets 9 is stacked can be lifted and lowered. The sheet leading end supporting unit 22 of the present invention that supports the leading end side of the bundle of sheets 9 can be lifted and lowered in Z1 direction which is shown in FIG. 2 with a lift driving unit 25. When the sheet is fed from the sheet stacking unit 6, the lift driving unit 25 drives the sheet leading end supporting unit 22 to move upward so that the upper surface of the bundle of sheets 9 gets in contact with the surface of the pick-up roller 30. When the sheet leading end supporting unit 22 is moved upward by the sheet stacking unit 6, the sheet rear end supporting unit 21 is integrally moved upward together with the sheet leading end supporting unit 22. However, this is given not for limitation. The sheet leading end supporting unit 22 may be moved separately from the sheet rear end supporting unit 21 and may be moved upward individually.

The pick-up roller 30 is arranged at upper side of the sheet leading end supporting unit 22. The pick-up roller 30 is supported at an upper position of the sheet leading end supporting unit 22 by a supporting member 31 which can be swung centering on a rotation shaft of a feeding roller 41 which is one of the pair of rollers 40. The pick-up roller 30 can be moved upward and downward in a direction shown with an arrow Z2 of FIG. 2. The position detecting sensor 38 that detects a position in the Z2 direction of the supporting member 31 is arranged near the supporting member 31. The position detecting sensor 38 is formed from a light sensor that has a light projecting unit and a light receiving unit, for example. The position detecting sensor 38 detects an amount of received light that varies depending on a position of the supporting member 31 to detect the position of the supporting member 31. The supporting member 31 swings in the arrow Z2 direction so that the pick-up roller 30 moves upward and downward integrally with the supporting member 31. The position detecting sensor 38 can be used as a sensor that detects a height position of the pick-up roller 30.

The supporting member 31 is provided with a pressing member 32 which is formed from a coil spring, for instance. The pressing member 32 pushes and presses the pick-up roller 30 downward. An end of the pressing member 32 is connected to the supporting member 31 and another end is connected to a wall surface which is fixed inside the image forming device 1. When the height position of the pick-up roller 30 changes, the pressing force by the pressing member 32 changes.

FIG. 3 illustrates a side-view of an exemplary structure of the sheet feeding device 5 when the sheet is fed by the pick-up roller 30. FIG. 3 shows an example when the sheet that has low rigidity such as a thin paper or a plain paper is stacked on the sheet stacking unit 6. When the sheet leading end supporting unit 22 moves upward, the pick-up roller 30 gets in contact with the top surface of the bundle of sheets 9 stacked on the sheet stacking unit 6 as illustrated in FIG. 3. The pick-up roller 30 then is rotated and driven in a predetermined rotation direction R1 by a motor which is not illustrated in FIG. 3. An elastic member that has an unevenness is used as a material or a surface of the pick-up roller 30. Once the pick-up roller 30 rotates in the rotation direction R1 while it is touching the top surface of the bundle of sheets 9, it can send at least one sheet which is placed at an upper part of the bundle of sheets 9 toward downstream side in the carrying direction.

The pair of rollers 40 are arranged at downstream side of the pick-up roller 30 in the carrying direction in which the sheet is carried. The pair of rollers 40 includes the feeding roller 41 and a separation roller 42. The pair of rollers 40 separates the first sheet and the second sheet or later when the multiple sheets are sent by the pick-up roller 30, and feeds the first sheet to the carrying path 45 at downstream side in the carrying direction. Also, the pair of rollers 40 restricts the second sheet or later moves to the carrying path 45 at downstream side in the carrying direction. The feeding roller 41 is rotated and driven in a certain rotation R2 direction by a motor which is not illustrated in FIG. 3. The separation roller 42 is in contact with the surface of the feeding roller 41. When the feeding roller 41 rotates in the rotation direction R2, the separation roller 42 rotates together in a rotation direction R3. The separation roller 42 includes a torque limiter. Unless the torque applied to the separation roller 42 becomes equal to or more than predetermined torque, the separation roller 42 does not rotate in the rotation direction R3.

When the single sheet is sent by the pick-up roller 30, for instance, toward the pair of rollers 40, the sheet is pinched by a nip part of the pair of rollers 40. The feeding roller 41 is in contact with the upper surface of the sheet and the separation roller 42 is in contact with the rear surface of the same sheet. When the feeding roller 41 rotates in the rotation direction R2, the sheet is carried to the carrying path 45 at downstream side of the pair of rollers 40. The torque equal to or more than predetermined torque is applied to the separation roller 42, and the separation roller 42 rotates in the rotation direction R3 in response to the movement of the sheet.

On the other hand, it is assumed that the multiple sheets are sent toward the pair of rollers 40 by the pick-up roller 30. In this case, the multiple sheets are pinched by the nip part of the pair of rollers 40. The feeding roller 41 is in contact with the surface of the first sheet which is placed at the top of the multiple sheets and the separation roller 42 is in contact with the rear surface of the second sheet or later. Under this condition, when the feeding roller 41 rotates in the rotation direction R2, the first sheet is carried to the carrying path 45 at downstream side in the carrying direction. The first sheet moves to the downstream side as it slides the surface of the second sheet so the second sheet or later does not move to downstream side. The torque equal to or more than the predetermined torque is not applied to the separation roller 42, so that the separation roller 42 does not rotate. The pair of rollers 40, thus, separates the first sheet from the second sheet or later and feeds the first sheet to downstream side in the carrying direction. The pair of rollers 40 also restricts the movement of the second sheet or later to downstream side in the carrying direction.

The separation member 50 is arranged between the pick-up roller 30 and the pair of rollers 40 in the carrying direction of sheets. When the sheet that has low rigidity such as a thin paper or a plain paper is fed, the separation member 50 touches the leading end of the sheet which is sent out by the pick-up roller 30 and restricts the number of sheets carried to the pair of rollers 40.

FIG. 4 illustrates an enlarged view of the pick-up roller 30, the pair of rollers 40 and the separation member 50. As illustrated in FIG. 4, the separation member 50 is arranged in an upper end part of the wall 23 that aligns the leading end of the bundle of sheets, for example. The separation member 50 includes a guide unit 51 that has an inclined surface, the height of which varies in the carrying direction, in the carrying path of the sheet between the pick-up roller 30 and the pair of rollers 40. The height of the inclined surface gradually gets higher toward downstream side from upstream side. When the sheet that has low rigidity such as a thin paper or a plain paper is fed by the pick-up roller 30, the separation member 50 limits the number of sheets carried toward the pair of rollers 40 with the guide unit 51. The separation member 50 may be formed integrally with the wall 23. The separation member 50 may also be formed separately and attached to the upper end part of the wall 23. When the separation member 50 is formed separately from the wall 23, the separation member 50 is formed by a metal plate or a PET (Polyethylene terephthalate) film, for example.

When the sheet that has low rigidity such as a thin paper or a plain paper is fed, as illustrated in FIG. 4, the height position of the lowest point of the pick-up roller 30 is kept at the lower position than an upper end position 52 of the separation member 50. When the sheet feeding operation is started, for instance, the feeding roller 41 idles in the rotation direction R2. Together with this, the supporting member 31 is lowered by a specified amount, and the height position of the pick-up roller 30 is lowered. The lift driving unit 25 gradually lifts the sheet leading end supporting unit 22 so that the upper surface of the bundle of sheets 9 is pressed to the pick-up roller 30. The lift driving unit 25 stops the lift of the sheet leading end supporting unit 22 at the point when the height position of the lowest point of the pick-up roller 30 reaches the first position which is lower than the upper end position 52 of the separation member 50 based on the position of the separation member 31 detected by the position detecting sensor 38. As a result, the pick-up roller 30 is in contact with the top surface of the bundle of sheets 9 at the lower position than the upper end position 52 of the separation member 50. The pressing member 32 presses the pick-up roller 30 downward with a certain pressing pushing force. A certain quantity of friction force is then generated between the pick-up roller 30 and the bundle of sheets 9. With this, the pick-up roller 30 is driven in the rotation direction R1 so that the sheet placed at the upper part of the bundle of sheets 9 is sent toward downstream side in the carrying direction.

When the multiple sheets are multifed toward downstream side by the pick-up roller 30, the separation member 50 can push the leading ends of the multiple sheets to the inclined surface of the guide unit 51. If the multiple sheets are moving toward downstream side, the multiple sheets, thus, receive the carrying resistance from the guide unit 51. The carrying resistance is generated so the separation member 50 only slides two or three sheets which are placed in an upper layer of the multiple sheets sent out by the pick-up roller 30 along the inclined surface of the guide unit 51, thus the two or three sheets are carried downstream. To be more specific, the angle of the inclined surface of the guide unit 51 is designed in advance to only enable two or three sheets in the upper layer to pass over the upper end position 52 of the guide unit 51 and move toward downstream side. The inclined surface of the guide unit 51 does not enable many sheets in the lower layer to pass over the upper end position 52 of the guide unit 51 due to the carrying resistance.

Hence, when the sheet that has low rigidity such as a thin paper or a plain paper is fed, the separation member 50 is enabled to be in contact with the leading end of the sheet sent out by the pick-up roller 30 and to restrict the number of sheets that is carried to the pair of rollers 40 to two to three. As a result, the multiple sheets are not carried to the nip part of the pair of rollers 40.

FIG. 5 illustrates a side view of the sheet feeding device 5 with the sheet stacking unit 6 on which the sheet that has high rigidity such as a thick paper is stacked. The thick paper of the present embodiment is a sheet that has a rigidity equal to or higher than a predetermined value. The thick paper is the sheet that is difficult to be normally fed when the pick-up roller 30 is at the first position. The thick paper excludes the thin paper and the plain paper. Once the sheet that has high rigidity such as the thick paper is stacked, the leading end of the sheet is placed on the sheet leading end supporting unit 22, and the rear end of the sheet is placed on the sheet rear end supporting unit 21. The center of the sheet is floated from the sheet stacking unit 6 as illustrated in FIG. 5. The sheet leading end supporting unit 22 is lifted in this status, and the pick-up roller 30 touches the upper surface of the sheet which is floating from the sheet stacking unit 6 as illustrated in FIG. 6. Once the sheet leading end supporting unit 22 is further lifted, the sheet is bent and the shape of which is changed, and pushes the pick-up roller 30 upward.

FIG. 7 illustrates an enlarged view when the height position of the pick-up roller 30 reaches the first position at which the sheet that has low rigidity such as a thin paper or a plain paper is fed. As illustrated in FIG. 7, when the pick-up roller 30 is positioned at the first position, the lowest point of the pick-up roller 30 is at a position lower than the upper end position 52 of the separation member 50 which is at downstream side. The sheet that has high rigidity such as a thick paper keeps somewhat being floated from the upper surface of the sheet leading end supporting unit 22. Once the pick-up roller 30 rotates in the rotation direction R1 from the status illustrated in FIG. 7, an angle of the sheet entering into the guide unit 51 of the separation member 50 is made bigger and the carrying resistance applied to the sheet from the separation member 50 becomes bigger. As a result, a leading end 9 a of the sheet cannot move toward the upper end position 52 of the guide unit 51 and stops. The sheet cannot be carried toward the pair of rollers 40.

The sheet feeding device 5 of the present embodiment changes the height position of the pick-up roller 30 depending on the type of the sheet stacked on the sheet stacking unit 6. When the sheet that has low rigidity such as a thin paper or a plain paper is stacked on the sheet stacking unit 6, the sheet feeding device 5, for example, determines the first position that makes the lowest height position of the pick-up roller 30 be lower than the upper end 52 of the separation member 50 as the height position of the pick-up roller 30. On the other hand, when the sheet that has low rigidity such as a thin paper or a plain paper is stacked on the sheet stacking unit 6, the sheet feeding device 5, for example, determines the lowest height position of the pick-up roller 30 to be at the second position which is higher than the first position. The sheet feeding device 5 lifts the sheet leading end supporting unit 22 to set the height position of the pick-up roller 30 to be the height position determined corresponding to the type of the sheet. The sheet feeding device 5 drives the pick-up roller 30 at the height position suitable for the type of the sheet and carries the sheet toward downstream side in the carrying direction.

FIG. 8 illustrates an enlarged view of the pick-up roller 30, the pair of rollers 40 and the separation member 50 when the sheet that has high rigidity such as a thick paper is fed. As illustrated in FIG. 8, when the sheet that has high rigidity such as a thick paper is to be fed, the sheet feeding device 5 drives the lift driving unit 25, and the sheet leading end supporting unit 22 pushes the pick-up roller 30 up. When the position of the lowest point of the pick-up roller 30 reaches the second position which is higher than the upper end 52 of the separation member 50, the sheet feeding device 5 stops lifting the sheet leading end supporting unit 22. As a result, the pick-up roller 30 is in contact with the top surface of the bundle of sheets 9 at the position higher than the upper end 52 of the separation member 50. A pressure value of the pressing member 32 becomes larger than that when the pick-up roller 30 is at the first position. The larger pressing force is generated by the pressing member 32 compared to that generated when the pick-up roller 30 is at the first position. Thus, the sheet that has high rigidity can be pressed to the upper surface of the sheet leading end supporting unit 22. The leading end 9 a of the sheet can be deformed to a posture almost parallel with the carrying direction. As the pressing force by the pressing member 32 becomes larger, a certain value of friction force can be generated between the pick-up roller 30 and the bundle of sheets 9. The pick-up roller 30 is rotated in the rotation direction R1 so that the sheet can be carried toward downstream side in the carrying direction.

As described above, when the pick-up roller 30 is hold at the second position, the sheet feeding device 5 rotates the pick-up roller 30 in the rotation direction R1 to send the sheet toward downstream side in the carrying direction. The sheet sent toward downstream side due to the rotation of the pick-up roller 30 is sent to downstream side at the position higher than the upper end 52 of the separation member 50. The leading end 9 a of the sheet is enabled to easily go over the upper end 52 of the separation member 50 and move to downstream side. As a result, the sheet feeding device 5 is enabled to feed the leading end 9 a of the sheet to the nip part of the pair of rollers 40.

As described above, the sheet feeding device 5 of the present embodiment determines the height position of the pick-up roller 30 for feeding the sheet depending on the type of the sheet stacked on the sheet stacking unit 6. For lifting and lowering the sheet leading end supporting unit 22, the sheet feeding device 5 drives the sheet leading end supporting unit 22 to set the height position of the pick-up roller 30 to be the height position determined based on the type of the sheet. The sheet feeding device 5 then rotates and drives the pick-up roller 30 to send the sheet toward downstream side in the carrying direction. Hence, the sheet feeding device 5 of the present embodiment corresponds to the type of the sheets varying from a thin paper to a thick paper. Whatever type of the sheet is used, the sheet feeding device 5 is enabled to supply the sheet to the carrying path 45 one by one. The sheet feeding device 5 ensures the feeding performance regardless of the sheet type.

The pressing member 32 that presses the pick-up roller 30 can apply larger pressing force when the pick-up roller 30 is at the second position compared to that is at the first position. The pick-up roller 30 is enabled to press the sheet to the sheet leading end supporting unit 22 when the pick-up roller 30 is at the second position so that the center of the sheet that has high rigidity such as a thick paper is not floated. As a result, the leading end of the sheet is arranged almost parallel to the carrying direction so that the angle of the sheet entering the separation member 50 is small. When the sheet is sent out by the pick-up roller 30, the rear surface of the sheet may be in contact with the guide unit 51 of the separation member 50. Even in this case, the sheet moves smoothly along the inclined surface of the guide unit 51, and moves toward downstream side over the upper end 52 of the separation member 50. As the pressing force by the pressing member 32 becomes larger, the larger friction force is generated between the pick-up roller 30 and the bundle of sheets 9 at the rotation of the pick-up roller 30. The pick-up roller 30, therefore, is enabled to send the sheet toward downstream side without failure.

FIGS. 9A, 9B and 9C illustrate an example of a change in the pressing force of the pressing member 32. As illustrated in FIG. 9A, for example, the pressing member 32 may generate a pressing force F1 when the pick-up roller 30 is at a first position P1, and a pressing force F2 when the pick-up roller 30 is at a second position P2. The pressing member 32 of this case is formed from a coil spring that has the same diameter in the shaft direction, for example.

As illustrated in FIG. 9B, the pressing member 32 may not generate any pressing force when the pick-up roller 30 is at the first position P1, and generate the pressing force F2 when the pick-up roller 30 is at the second position P2. In this case, one end of the pressing member 32, for example, can be separated from the supporting member 31. When the pick-up roller 30 is at the first position P1, the pressing member 32 becomes the natural length and is separated from the supporting member 31. In the movement of the pick-up roller 30 from the first position P1 the second position P2, the pressing member 32 and the supporting member 31 are in contact with each other. As the pick-up roller 30 further lifts, the pressing member 32 is contracted and the pressing force gradually increases. When the pick-up roller 30 moves to the second position P2, the pressing member 32 may press the pick-up roller 30 with the pressing force F2.

If it is necessary to increase the pressing force F2 generated when the pick-up roller 30 is at the second position, the pressing member 32 that increases the pressing force nonlinearly may be used as illustrated in FIG. 9C. In this case, a conical coil spring, for example, can be used as the pressing member 32 so that the pressing force is increased nonlinearly. When the pick-up roller 30 moves to the second position P2 to feed the sheet that has high rigidity such as a thick paper, the pressing member 32 is enabled to press the pick-up roller 30 to the bundle of sheets 9 with the large pressing force F2. As a result, the carrying failure due to the pick-up roller 30 is reduced.

A control structure that controls the sheet feeding device 5 as described above is explained next. FIG. 10 illustrates a block diagram showing an example of a control structure of the image forming device 1. The image forming device 1 includes a controller 60, a sheet carrier 71 and a communication interface 70 in addition to the aforementioned image forming unit 2, the operational panel 15, the sheet stacking unit 6 and the position detecting sensor 38.

The operational panel 15 is constructed by a device such as a color liquid crystal display, for instance. The operational panel 15 includes a display unit 15 a and a manipulation unit 15 b. Various kinds of screens operable for a user are displayed on the display unit 15 a. The manipulation unit 15 b is constructed by a part such as a touch panel key. The manipulation unit 15 b receives an input by the user.

The sheet carrier 71 carries the sheet stacked on the sheet stacking unit 6 along the carrying path 45. The sheet carrier 71 includes a roller driving part 72 that rotates and drives the feeding roller 41 and the pick-up roller 30. The roller driving part 72 is formed from a motor, for example.

The communication interface 70 is to establish communication with another device over a network such as LAN (Local Area Network).

The controller 60 controls operations of each part included in the image forming device 1. The controller 60 includes a CPU 61 and a memory 62. The controller 60 reads and executes a program 63 stored in the memory 62. The memory 62 is a non-volatility storage in which the program 63 is stored. A variety of data other than the program 63 can be stored in the memory 62.

The CPU 61 of the controller 60 executes the program 63 so that it serves as a job controller 64. The job controller 64 controls the processing of the job in the image forming device 1. The job controller 64 displays a setting screen of the job on the display unit 15 a of the operational panel 15, and accepts an operation for setting the job from the user. The job controller 64 configures the setting of the job based on the user's operations of the setting.

The job controller 64 is also enabled to receive a print job sent from another device via the communication interface 70 and control processing of the print job. Information relating to the setting of the job is attached to the print job, and the job controller 64 configures the setting of the print job based on the information.

When the bundle of the sheets 9 is stacked on each of the multiple feeding trays 3 a, 3 b, 3 c and 3 d, and the manual bypass tray 4, the job controller 64 displays the screen to input the type of the sheet on the display unit 15 a, and accepts the operation to specify the type of the sheet from the user. The job controller 64 then stores the type of the sheet stacked corresponding to each of the multiple feeding trays 3 a, 3 b, 3 c and 3 d, and the manual bypass tray 4 in the memory 62.

When the instructions to start the processing of the job is given by the user, or the print job is received, the job controller 64 starts the processing of the job. It is assumed, for example, the manual bypass tray 4 (the sheet feeding device 5) is specified as the tray to feed the sheet according to the setting of the job. In this case, the job controller 64 brings the sheet stacking unit 6, the sheet carrier 71 and the image forming unit 2 into operation to control the processing of the job specified by the user. The job controller 64 serves as a job type determining part 65 and the position adjusting part 66.

The job type determining part 65 determines the type of the sheet stacked on the manual bypass tray 4 which is specified as the tray from which the paper is fed. To be more specific, the job type determining part 65 reads the information related to the type of the sheet stacked on the manual bypass tray 4 from the memory 62 and determines the type of the sheet based on the information. The job type determining part 65 thus is enabled to identify if the sheet fed from the manual bypass tray 4 is any of the thin paper, the plain paper or the thick paper upon the start of the job processing.

The position adjusting part 66 determines the height position of the pick-up roller 30 based on the result of the determination carried out by the job type determining part 65, and drives the lift driving unit 25. More specifically, the position adjusting part 66 determines the height position of the pick-up roller 30 at the position suitable for the type of the sheet fed from the manual bypass tray 4, and drives the lift driving unit 25 to set the position of the lowest point of the pick-up roller 30 to be at the determined position. When the sheet fed from the manual bypass tray 4 is the sheet that has low rigidity such as a thin paper or a plain paper, the position adjusting part 66 determines the first position that sets the lowest point of the pick-up roller 30 to be lower than the upper end 52 of the separation member 50. The position adjusting part 66 detects the height position of the pick-up roller 30 based on the output from the position detecting sensor 38 and drives the lift driving unit 25. The position adjusting part 66 thus adjusts the height position of the lowest point of the pick-up roller 30 to be the first position. When the sheet that has high rigidity such as a thick paper is fed from the manual bypass tray 4, the position adjusting part 66 determines the second position that sets the lowest point of the pick-up roller 30 be higher than the upper end 52 of the separation member 50. The position adjusting part 66 detects the height position of the pick-up roller 30 based on the output from the position detecting sensor 38 and drives the lift driving unit 25. The position adjusting part 66 thus adjusts the height position of the lowest point of the pick-up roller 30 to be the second position.

As described above, the height position of the pick-up roller 30 is adjusted according to the type of the sheet. The job controller 64 then drives the sheet carrier 71 to start the feeding operation of the sheet stacked on the sheet stacking unit 6.

A process sequence performed by the controller 60 is explained next. FIG. 11 illustrates a flow diagram explaining an exemplary procedure of the process that controls operations of the sheet feeding device 5. After detecting that the sheet is stacked on the sheet stacking unit 6 (step S1), the controller 60 determines the type of the sheet (step S2). The controller 60 displays the screen for inputting the type of the sheet on the display unit 15 a of the operational panel 15, and accepts the user's operation that specifies the type of the sheet.

FIGS. 12A, 12B and 12C illustrate examples of the screens displayed on the display unit 15 a of the operational panel 15. Button images B1 and B2 for the user to select either a thin paper or the plain paper, or a thick paper are displayed on a screen G1 of FIG. 12A. When the user stacks the thin paper or the plain paper on the sheet stacking unit 6, he or she touches the button image B1 so that the thin paper or the plain paper is set as the sheet type. When the user stacks the thick paper on the sheet stacking unit 6, he or she touches the button image B2 so that the thick paper is set as the sheet type.

A screen G2 of FIG. 12B may be displayed on the display unit 15 a. The screen G2 asks the user to input the rigidity of the sheet. An input field B3 for inputting a value of the rigidity of the sheet is displayed in the center of the screen. The user inputs the rigidity of the sheet in the input field B3 so that the type of the sheet can be set.

A screen G3 of FIG. 12C may be displayed on the display unit 15 a. The screen G3 asks the user to input a basis weight of the sheet. An input field B4 for inputting a value of the basis weight of the sheet is displayed in the center of the screen. The user inputs the basis weight of the sheet in the input field B4 so that the type of the sheet can be set. The basis weight of the sheet indicates a value of the sheet weight per unit area. The basis weight of the sheet and the rigidity of the sheet have a correlation between each other.

The controller 60 determines the type of the sheet based on the user operation to the screen G1, G2 or G3 as described above, and identifies whether the type of the sheet stacked on the sheet stacking unit 6 is either the thin paper or the plain paper, or the thick paper. When the user inputs the rigidity of the sheet, for example, the controller 60 determines the sheet is the thick paper if the rigidity of the sheet is equal to or higher than a predetermined value. If the rigidity of the sheet is lower than a predetermined value, the controller 60 determines the sheet is the thin paper or the plain paper. When the user inputs the basis weight of the sheet, the controller 60 determines the sheet is the thick paper if the basis weight of the sheet is equal to or higher than a predetermined value. If the basis weight of the sheet is lower than a predetermined value, the controller 60 determines the sheet is the thin paper or the plain paper.

Referring back to the flow diagram of FIG. 11, when the controller 60 determines the sheet stacked on the sheet stacking unit 6 is the thin paper or the plain paper (when a result of step S3 is YES), it determines the height position of the pick-up roller 30 for feeding the sheet to be the first position which is the lower position than the upper end 52 of the separation member 50 (step S4). When the controller 60 determines the sheet stacked on the sheet stacking unit 6 is the thick paper (when a result of step S3 is NO), it determines the height position of the pick-up roller 30 for feeding the sheet to be the second position which is the higher position than the upper end 52 of the separation member 50 (step S5).

After detecting the instruction to start the processing of the job (step S6), the controller 60 drives the lift driving unit 25, and starts lifting the sheet leading end supporting unit 22 (step S7). The controller 60 determines if the height of the pick-up roller 30 reaches the height position determined at step S4 or S5 based on the input from the position detecting sensor 38 (step S8). If the height position of the pick-up roller 30 hasn't reached (when a result of step S8 is NO), the controller 60 keeps lifting of the sheet leading end supporting unit 22. When the height of the pick-up roller 30 reaches the height position determined at step S4 or S5 (when a result of step S8 is YES), the controller 60 stops driving the lift driving unit 25, and holds the pick-up roller 30 at the height position determined at step S4 or S5 (step S9). The controller 60 drives the sheet carrier 71, and starts the feeding operation of the sheet stacked on the sheet stacking unit 6 (step S10).

When the sheet fed from the sheet stacking unit 6 is the thick paper, the lowest point of the pick-up roller 30 is at the higher position than the upper end 52 of the separation member 50. The sheet goes over the upper end 52 of the separation member 50 smoothly and is carried toward the pair of rollers 40.

When the sheet fed from the sheet stacking unit 6 is the thin paper or the plain paper, the lowest point of the pick-up roller 30 is at the lower position than the upper end 52 of the separation member 50. When multiple sheets are fed by the pick-up roller 30 toward downstream side, the guide unit 51 of the separation member 50 is enabled to limit the number of the sheets carried toward the pair of rollers 40. The pair of rollers 40 is enabled to separate normally the first sheet placed on the top surface from the second sheet or later. Only the first sheet is then carried to downstream side of the carrying path 45.

As described above, the sheet feeding device 5 of the present embodiment includes a sheet stacking unit 6, the pick-up roller 30, the pair of rollers 40 and the separation member 50. The sheet stacking unit 6 can be lifted up and lowered on which the bundle of sheets 9 is stacked. The pick-up roller 30, that is in contact with the top surface of the bundle of sheets 9 and can be moved up and down in accordance with the movement of the sheet stacking unit 6, sends the sheet of the bundle of sheets 9 toward downstream side in the carrying direction. The pair of rollers 40 is placed in downstream side in the carrying direction. When the multiple sheets are sent out by the pick-up roller 30, the pair of rollers 40 separates the first sheet from the second sheet or later, then feeds the first sheet toward downstream side in the carrying direction. The pair of rollers 40 also restricts the movement of the second sheet or later. The separation member 50 is arranged between the pick-up roller 30 and the pair of rollers 40. The separation member 50 is in contact with the leading end of the sheet send out by the pick-up roller 30 and limits the number of the sheet carried toward the pair of rollers 40. The sheet feeding device 5 determines the height position of the pick-up roller 30 depending on the type of the sheet stacked on the sheet stacking unit 6, and lifts and lowers the sheet stacking unit 6 to be at the determined height position. As a result, when the sheet stacked on the sheet stacking unit 6 is to be fed, the height position of the pick-up roller 30 is adjusted to the position suitable for the type of the sheet and the sheet can be fed. The sheet feeding device 5 corresponds to the type of the sheets varying from a thin paper to a thick paper. Whatever the type of the sheet is to be used, the feeding operation can be carried out normally. The feeding efficiency is ensured regardless of the type of the sheet.

When the sheet stacked on the sheet stacking unit 6 is the thick paper, the sheet feeding device 5 determines the height position of the pick-up roller 30 to be the higher position than that for feeding the thin paper or the plain paper. Malfunctions that the sheet is not carried toward downstream side of the separation member 50 when the sheet is the thick paper can be solved.

The sheet feeding device 5 also includes the pressing member 32 that presses the pick-up roller 30 to the bundle of sheets 9 stacked on the sheet stacking unit 6. The pressing force by the pressing member 32 generates the predetermined frictional force between the pick-up roller 30 and the sheet when the pick-up roller 30 sends the sheet toward downstream side in the carrying direction. The idle rotation of the pick-up roller 30, therefore, can be prevented and the sheet can be sent toward downstream side in the carrying direction without failure.

The pressing member 32 is arranged to make the pressing force generated when the pick-up roller 30 is at the second position be larger than the pressing force generated when the pick-up roller 30 is at the first position. When the sheet is the thick paper, the friction force between the pick-up roller 30 and the sheet becomes larger, so that the thick paper can be sent toward downstream side in the carrying direction without failure.

When the sheet feeding device 5 is arranged as the manual bypass tray 4 as described in the present embodiment, the sheet stacking unit 6 has the inclined surface that makes the height position become higher from the leading end position to the rear end position. When the sheet that has the high rigidity such as the thick paper is stacked, the center of the sheet is floated. Even in this condition, once the pick-up roller 30 lifts to the second position, the pressing force by the pressing member 32 becomes larger so that the center of the sheet is pressed to the bottom surface of the sheet stacking unit 6. The predetermined friction force thus can be obtained between the pick-up roller 30 and the sheet. The pressing force by the pressing member 32 generated when the pick-up roller 2 is at the second position becomes larger than that generated when the pick-up roller 2 is at the first position. The friction force necessary for carrying the sheet, therefore, can be generated.

Although the embodiments of the present invention have been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and not limitation, the scope of the present invention should be interpreted by terms of the appended claims

(Modifications)

While the preferred embodiment of the present invention has been described above, the present invention is not limited to the preferred embodiment. Various modifications may be applied to the present invention.

The image forming device 1 of the above-described embodiment is constructed by one of MFPs. However, this is given not for limitation. The image forming device 1 does not always have to be one of MFPs. The image forming device 1 may be a printer that only includes the print function.

In the above-described preferred embodiment, the manual bypass tray 4 serves as the sheet feeding device 5. The tray to which the above-described structure can be applied as the sheet feeding device 5 does not have to be the manual bypass tray 4. The aforementioned structure of the sheet feeding device 5 may be applied to the multiple feeding trays 3 a, 3 b, 3 c and 3 d that are incorporated in the device body of the image forming device 1, for example. The aforementioned structure of the sheet feeding device 5 may be applied to the document tray 12 included in the automatic document feeder 11.

In the above-described preferred embodiment, the type of the sheet stacked on the sheet stacking unit 6 is input by the user and the controller 60 determines the type of the sheet based on the information input by the user. However, this is given not for limitation. The type of the sheet can be automatically detected with a light reflection type sensor or a light transmission type sensor. For using the light reflection type sensor, for example, the light reflection type sensor is arranged above the sheet stacking unit 6. The light reflection type sensor irradiates a light to an upper surface of the bundle of sheets 9 and detects a reflection light from the upper surface of the bundle of sheets 9. As a result, the attenuated light amount based on a thickness of the sheet can be detected, and the basis weight can be detected based on the thickness of the sheet. Hence, the type of the sheet can be determined from the result of the detection of the reflection light.

In the above-described preferred embodiment, in order to detect the height position of the pick-up roller 30, the position detecting sensor 38 is arranged and the position of the supporting member 31 is detected. The position detecting sensor 38 may be a combination of two sensors, the first sensor and the second sensor. When the pick-up roller 30 is at the first position, the first sensor is powered on and when the pick-up roller 30 separates from the first position, the first sensor is powered off. When the pick-up roller 30 is at the second position, the second sensor is powered on and when the pick-up roller 30 separates from the second position, the second sensor is powered off. The position detecting sensor 38 does not always have to detect the position of the supporting member 31. The position detecting sensor 38, for example, may be arranged to directly detect the position of the pick-up roller 30.

In the above-described preferred embodiment, the second position that is determined as the height position of the pick-up roller 30 when the sheet stacked on the sheet stacking unit 6 is the thick paper is higher than the upper end 52 of the separation member 50. The second position does not have to be higher than the upper end 52 of the separation member 50. The second position, for example, may be the height position near the upper end 52 of the separation member 50. More specifically, the second position may be the same height position as the upper end 52 of the separation member 50 or slightly lower position than the upper end 52 of the separation member 50.

Furthermore, in the above-described preferred embodiment, there are two types, the thick paper and plain paper, and the thick paper for the types of the sheet. The types of the sheet determined by the controller 60 are not limited to two types. When the sheet is the thick paper, for instance, the controller 60 may determine the type of the thick paper corresponding to the rigidity and/or the basis weight, and determine the height position (the second position) of the pick-up roller 30 based on the type of the thick paper. In this case, as the rigidity and/or the basis weight has the higher value, the controller 60 preferably determines the higher position as the second position. 

What is claimed is:
 1. A sheet feeding device, comprising: a sheet stacking unit that is enabled to be lifted and lowered while a bundle of sheets is stacked; a pick-up roller that is in contact with a top surface of the bundle of sheets and is enabled to move up and down in accordance with the lifting and lowering movement of the sheet stacking unit, and that sends a sheet of the bundle of sheets toward downstream side in a carrying direction; a pair of rollers that is arranged at downstream side in the carrying direction of the pick-up roller, and when multiple sheets are sent out by the pick-up roller, that separates the first sheet from the second sheet or later and feeds the first sheet toward downstream side in the carrying direction, and restricts a movement of the second sheet or later; a separation member that is arranged between the pick-up roller and the pair of rollers and that is in contact with a leading end of the sheet that is sent out by the pick-up roller and limits the number of the sheet carried to the pair of rollers; and a hardware processor that determines a height position of the pick-up roller corresponding to a type of the sheet stacked on the sheet stacking unit and lifts and drives the sheet stacking unit to set the pick-up roller to be at the determined height position.
 2. The sheet feeding device according to claim 1, wherein the hardware processor determines the type of the sheet based on a rigidity or a basis weight of the sheet.
 3. The sheet feeding device according to claim 1, wherein the hardware processor determines the height position of the pick-up roller to be higher than the height position for a thin paper or a plain paper when the sheet stacked on the sheet stacking unit is a thick paper.
 4. The sheet feeding device according to claim 1, further comprising: a pressing unit that presses the pick-up roller to the bundle of sheets stacked on the sheet stacking unit.
 5. The sheet feeding device according to claim 1, wherein the separation member includes an inclined guide unit, the height of which is gradually elevated from upstream side to downstream side of a sheet carrying path between the pick-up roller and the pair of rollers, and the hardware processor further: determines the height position of the pick-up roller to be the first position which is lower than an upper end of the guide unit when the sheet stacked on the sheet stacking unit is the thin paper or the plain paper; and determines the height position of the pick-up roller to be the second position which is higher than the first position and near the upper end position of the guide unit when the sheet stacked on the sheet stacking unit is the thick paper.
 6. The sheet feeding device according to claim 5 further comprising: a pressing unit that presses the pick-up roller to the bundle of sheets stacked on the sheet stacking unit, wherein the pressing unit generates a larger pressing force when the pick-up roller is at the second position compared to the pressing force generated when the pick-up roller is at the first position.
 7. The sheet feeding device according to claim 6, wherein the pressing unit does not press the pick-up roller when the pick-up roller is at the first position, and presses the pick-up roller when the pick-up roller is at the second position.
 8. The sheet feeding device according to claim 4, wherein the pressing unit increases the pressing force nonlinearly as the height position of the pick-up roller becomes higher.
 9. The sheet feeding device according to claim 4, wherein the pressing unit is formed from a coil spring.
 10. The sheet feeding device according to claim 1, wherein the sheet stacking unit has an inclined surface, the height position of which becomes higher from a leading end position to a rear end position of the sheet.
 11. An image forming device that includes the sheet feeding device according to claim
 1. 